Metal surface coating methods



United States atent 3,459,604 METAL SURFACE COATING METHODS Dennis Brian Freeman, Harrow, Middlesex, and Colm Anthony Gill, London, England, assignors to Hooker Chemical Corporation, Niagara Falls, N.Y., a corporation of New York No Drawing. Filed Apr. 18, 1966, Ser. No. 543,040

Int. Cl. C23f 7/26 US. Cl. 148-624 7 Claims ABSTRACT OF THE DISCLOSURE A coating composition useful in forming lubricant carrying coatings on chromium-containing metal surfaces which is an aqueous solution containing oxalic acid and citric acid, the weight ratio of oxalic acid to citric acid being at least 0.221 but not substantially in excess of about 50:1, and the oxalic acid being present in an amount of from about 10 to 120 grams per liter.

This invention relates to an improved method for coating metal surfaces and more particularly it relates to improvements in compositions and processes for forming oxalate coatings on chromium-containing metal surfaces.

Compositions and processes for forming oxalate coatings on metal surfaces, and particularly chromium-containing surfaces, such as stainless steel, are well-known in the art. Specifically, the compositions used for forming these coatings are aqueous acidic solutions of oxalic acid, which solutions frequently also contain an activator material, as well as one or more accelerator materials. The metal surfaces to be coated are contacted with the oxalate coating solutions for a period of time suflicient to effect the formation of the desired oxalate coating on the metal. The thus-produced oxalate coatings are particularly useful on surfaces which are to be subjected to a cold forming operation, as the oxalate coatings have been found to be highly effective as a lubricant carrier and/or a parting layer during the deforming operation.

Although high quality oxalate coatings, having good adhesion and lubricant carrying properties, are initially produced, difliculties have sometimes been experienced as the coating solutions are used over a prolonged period of time. It has been found that as metal is processed through the oxalate solution, there is a gradual deterioration in the quality of the coatings produced and in some instances, a point is reached where coatings are no longer obtained. When this happens, it is then necessary to discard the entire coating solution and build up a new one. This, of course, is both time consuming and costly.

It is, therefore, an object of the present invention to provide composition and process for forming an oxalate coating on chromium-containing metal surfaces.

A further object of the present invention is to provide an improved composition and process for forming oxalate coatings on chromium containing metal surfaces, wherein the compositions and processes may be used over an extended period of time without deterioration of the oxalate coating produced.

These and other objects will become apparent to those skilled in the art from the description of the invention which follows.

Pursuant to the above objects, the present invention includes an aqueous coating solution, suitable for forming coatings on chromium-containing surfaces, which coating solution comprises oxalic acid and citric acid, the citric acid being present in an amount suflicient to provide a weight ratio of oxalic acid to citric acid of at least 0.2: 1. It has been found that by including such amounts of citric 3,459,504 Patented Aug. 5, 1969 acid in the oxalate coating solution, the useful life of these solutions is greatly prolonged.

More specifically, the coating compositions of the present invention desirably contain the oxalic acid and citric acid in a weight ratio within the range of about 3 to 20:1. Additionally, although sufiicient citric acid should be added to the oxalic acid solutions to obtain the desired improvement in useful life of the solutions, it has been found that in most instances, the ratio of oxalic acid to citric acid should not be greater than about 50:1.

In addition to the oxalic acid and citric acid, the oxalate coating solutions of the present invention may also contain an activating compound and one or more accelerating compounds. Exemplary of the activators which ma be included in the composition are various halide compounds, such as the alkali metal and ammonium halides, and in particular, the alkali metal and ammonium chlorides and fluorides. Additionally, the halogen acids, and particularly hydrochloric acid and hydrofluoric acid are also suitable as activator materials in these oxalate coating solutions.

It is to be appreciated, that as used in the specification and claims, the term halide is intended to include the chlorides, bromides, iodides, and fluorides and the term alkali metal is intended to include sodium, potassium, lithium, cesium and rubidium. Of these, the preferred halides are the chlorides and fluorides and the preferred alkali metals are sodium and potassium. Accordingly, hereinafter primary reference will be made to these materials.

Various accelerator compounds as are known in the art, may be included in the oxalate coating solutions. Exemplary of those which may be used are organic nitro compounds, such as those described in British patent specification 769,779. Exemplary of specific preferred compounds of this type which may be used are picric acid, nitroguanadine, p-nitroaniline, m-nitrobenzene sulfonic acid, and the like.

Additionally, sulfur-containing accelerators may also be used in the oxalate coating solutions of the present invention. Examples of suitable accelerators of this type include the alkali metal and ammonium thiosulfates, sulfites, thiocyanates and the like. Additionally, accelerator materials which are comprised of solutions of sulfur dioxide in aqueous ammonia, such as are described in British patent specification 886,594, have been found to be particularly useful.

As has been noted hereinabove, the treating solutions of the present invention are aqueous solutions containing oxalic acid. Desirably, in these solutions, the oxalic acid is present in amounts within the range of about 10 to about grams per liter. Additionally, where halide activators are included in this solution they are desirably present in amounts within the range of about 5 to '50 grams per liter. The sulfur-containing accelerator materials are desirably present in amounts of about 1 to 10 grams per liter while the organic nitro compound accelerators are desirably present in amounts up to about 20 grams per liter, with amounts within the range of about 1 to 10 grams per liter being preferred. Additionally, as has been heretofore noted, the ratio of oxalic acid to citric acid is desirably within the range of 3 to 20:1.

The oxalate coating solutions of the present invention may be applied to a variety of chromium-containing metal surfaces. Thus, the solutions may be used to treat chromium-containing nickel alloys, and particularly those which should be substantially free of scale or other contaminating materials. Accordingly, if scale is contained on the surface, it should first be descaled or degreased, if oily deposits are on the surface. The subject oxalate coating solutions may be applied to the cleaned chromium-containing metal surfaces by various means, including immersion, spraying, flowing, or the like. Desirably, the solutions are at an elevated temperature, e.g., 45 to 85 centigrade and contact times of from about 2 to 15 minutes are typical. It is believed that those in the art will readily be able to determine the solution concentrations, temperatures, application times and techniques, which are suitable in each instance to obtain the desired oxalate coating on the metal surface.

In order that those skilled in the art may better understand the present invention and the manner in which it may be practiced the following specific examples are given. In these examples, unless otherwise indicated, temperatures are in degrees centigrade and parts and percents are by weight. It is to be appreciated, however, that these examples are merely exemplary of the present invention and are not to be taken as a limitation thereof.

Example 1 An aqueous coating solution was formulated containing 48 grams per liter oxalic acid, 30 grams per liter ammonium chloride, 2.4 grams per liter ammonium sulfite, and 16 grams per liter citric acid. This solution had a weight ratio of oxalic acid to citric acid of 3:1. A series of stainless steel articles were immersed in this solution for minutes, at a temperature of about 70 degrees centigrade to form a satisfactory oxalate coating on the metal surface. It was noted that as the stainless steel articles were coated with the solution, there was a gradual buildup of heavy metal ions in the coating solution, which ions were principally ferric and chromic ions. Notwithstanding this, however, satisfactory oxalate coatings were still formed on the stainless steel articles even after the buildup of heavy metal ion in the solution had reached three grams per liter.

Example 2 By way of comparison, a similar oxalate coating bath as in Example 1 was formed with the exception that the bath contained no citric acid. As in Example 1, a series of stainless steel articles were coated in the bath by immersion for 10 minutes at a temperature of about 70 degrees centigrade. As with the bath of Example 1, there was a gradual buildup of heavy metal ion, principally ferric and chromic ions, in the bath as the stainless steel articles were coated. As this buildup of heavy metal ions continued there was a gradual deterioration of the oxalate coatings formed on the surface to the point that when the heavy metal ion concentration reached three grams per liter, no oxalate coatings were formed on the stainless steel articles.

While there have been described various embodiments of the invention, the methods and compositions described are not intended to be understood as limiting the scope of the invention, as it is realized that changes therewithin are possible and it is further intended that each element recited in any of the following claims is to be understood as referring to all equivalent elements for accomplishing substantially the same results in substantially the same or equivalent manner, it being intended to cover the invention broadly in whatever form its principle may be utilized.

What is claimed is:

1. A coating composition, useful in forming lubricant carrying coatings on chromium-containing metal surfaces, which comprises an aqueous solution containing citric acid and from about 10 to grams per liter of oxalic acid, the weight ratio of oxalic acid to citric acid being at least 0.2:1 but not substantially in excess of about 50:1.

2. The composition as claimed in claim 1 wherein the aqueous coating solution also contains a halide activator selected from alkali metal halides, ammonium halides and halogen acids, the halide activator being present in an amount of from about 5 to 50 grams per liter, and at least one accelerator which is a sulfur containing compound selected from alkali metal thiosulfates, alkali metal sulfites, alkali metal thiocyanates, ammonium thiosulfates, ammonium sulfites, ammonium thiocyanates, and solutions of sulfur dioxide in aqueous ammonia, or an organic nitro compound selected from picric acid, nitroguanidine, p-nitroaniline, and m-nitrobenzene sulfonic acid, the sulfur containing compound accelerator being present in an amount of from 1 to 10 grams per liter and the organic nitro compound accelerator being present in an amount of up to about 20 grams per liter.

3. The composition as claimed in claim 2 wherein the weight ratio of oxalic acid to citric acid is within the range of about 3-2021.

4. A process for forming oxalate coatings on chromiumcontaining surfaces which comprises contacting a chromium-containing surface with the aqeuous coating solution as claimed in claim 1 and maintaining said aqueous solution in contact with the chromium containing surface for a period sufiicient to form the desired oxalate coating on the surface.

5. The process as claimed in claim 4 wherein the aqueous coating solution also contains a halide activator selected from alkali metal halides, ammonium halides and halogen acids, the halide activator being present in an amount from about 5 to 50 grams per liter, and at least one accelerator which is a sulfur-containing compound selected from alkali metal thiosulfates, alkali metal sulfites, alkali metal thiocyanates, amomnium thiosulfates, ammonium sulfites, ammonium thiocyanates, and solutions of sulfur dioxide in aqueous ammonia, or an organic nitro compound selected from picric acid, nitroguanidine, p-nitroaniline, and m-nitrobenzene sulfonic acid, the sulfur-containing compound accelerator being present in an amount of from about 1 to 10 grams per liter and the organic nitro compound accelerator being present in amounts u to about 20 grams per liter.

6. The process as claimed in claim 5 wherein the weight ratio of oxalic acid to citric acid in the coating solution is within the range of about 3-20:l.

7. The process as claimed in claim 6 wherein the chromium-containing surface which is contacted with the aqueous coating solution is a stainless steel surface.

References Cited UNITED STATES PATENTS 2,577,887 12/1951 Gibson 1486.l4 2,800,421 7/1957 Goodspeed l486.l4

RALPH S. KENDALL, Primary Examiner U.S. Cl. X.R. 1486 

